Modulated carrier wave signaling system



Patented June 29, 1937 PATENT OFFICE MODULATED CARRIER WAVE SIGNALINGSYSTEM William Theodore Ditcham, Chelmsford, England,

assignor to Radio Corporation of America, a corporation of DelawareApplication February 15, 1933, Serial No. 656,869 In Great Britain March22, 1932 10 Claims.

This invention relates to radio and other high frequency modulatedcarrier wave systems, and has for its object to provide an improvedsystem suitable for use for broadcasting purposes and in other caseswhere a wide range of degree of modulation is likely to be required. Inbroadcast and other similar high frequency modulated carrier systems itis usual to modulate the output from a low power oscillator or low powerhigh frequency amplifier and to amplify the resultant modulated highfrequency by means of an amplifier of high power, the amplifiedmodulated oscillations then being transferred to the aerial or othertransmitting device proper. Where, however, the de- 5 gree of modulationlikely to be required is liable to vary between fairly wide limits, forexample in a broadcasting system where the modulation may be Very weakat one moment and very strong at the next, such systems present thepractical disadvantage that since the anode and grid voltages applied tothe valve or valves in the high power amplifier have to be so adjustedthat the necessary increments in voltage due to modulation, influencethe power output of the said valve or valves in a linear manner, theefficiency of the system when the carrier is unmodulated is necessarilyvery low. To put the matter in another way, since the power amplifiermust amplify linearly the full range of voltage input thereto, thevoltages which may be applied to said power amplifier when the carrieris unmodulated are necessarily well below the best values for givinghigh efliciency.

The principal object of the present invention is to avoid thisdiificulty, and according to the invention there are provided two highfrequency high power amplifiers in effective parallel relationship inthe transmission channel one of these amplifiers being so adjusted andarranged as to give a varying power output corresponding to modulatedcarrier variations above the carrier level while the other is soadjusted and arranged as to give a varying power output corresponding tomodulated carrier variations below the carrier level, the total poweroutput corresponding to variations in modulated carrier over the wholerange of modulation.

The input to the two high power amplifiers may be a modulated carrierwave or the power amplifiers may be driven by unmodulated high frequencyand modulation efiected in the amplifiers themselves. In the latter casetwo modulating circuits are provided, one for each high power amplifier,and the whole arrangement and conditions of adjustment are such that onecombination of amplifier and associated modulating circuit deals withmodulation of carrier wave above the carrier level and the other withmodulation of carrier wave below the carrier level.

The invention is illustrated in the accompanying drawing which showsdiagrammatically two arrangements in accordance therewith.

Fig. 1 shows a schematic circuit diagram of one form of my invention;and

Fig. 2 shows a circuit diagram of another embodiment of my invention.

Referring first to Figure 1 which shows one way of carrying out theinvention, a radio transmitter comprises a master or drive oscillator Iwhich drives a low power high frequency amplifier 2, to which modulationfrom a speech, music, or other system 2a is applied as in the usual way,the output from this low power amplifier thus consisting of a modulatedhigh frequency carrier. There are provided two relative high poweramplifiers 3 and 4 which are in effective parallel relationship in thetransmission channel. For the sake of convenience in description one ofthese amplifiers 4 will be referred to as the limiter amplifier and theother 3 as the quiescent amplifier. The quiescent power amplifier 3 isso adjusted as regards negative grid bias and applied alternating gridvoltage that at the carrier level the said amplifier is almost or quitequiescent so that in these circumstances the power output of thisamplifier is small or even zero. The limiter amplifier 4 comprises or isassociated with a limiting device 5 whereby the output of this amplifieris prevented from rising above the carrier level notwithstanding anyincrement of the output from the low power amplifier 2 during thepositive half cycle of modulation. The limiting device 5 may consist ofany known form of voltage limiter connected in series or in parallelwith or coupled to the grid circuit of the limiter amplifier, or it maybe an arrangement whereby the negative grid bias applied to the limiteramplifier 4 is au tomatically increased as the applied driving voltageincreases. Suitable decoupling means 6, such as any well known bridgearrangement for preventing reaction between circuits, are arrangedbetween the output circuits of the amplifiers 3 and 4 and the twoamplifiers are arranged to feed their output energy in parallel to anaerial or other load circuit 1. Anode voltage for the two high frequencyamplifiers may be provided from separate sources, but is preferablyprovided by the same source I I' connected as shown, and preferably alsothis source includes means, such as a large capacity condenser l2capable of handling large instantaneous overloads.

It will be seen that in operation and when the carrier is unmodulated,the limiter amplifier will be fully driven and will deliver power to theaerial at good efiiciency, while the quiescent amplifier will bedelivering substantially no power and taking substantially no power fromthe source of supply. During the positive cycle of modulation the lowpower amplifier 2 will provide increased driving voltages to each of thetwo high power amplifiers 3 and 4, but owing to the effect of thelimiting device in the limiter amplifier the increased driving voltagethereto will have no effect on the power output thereof. The quiescentamplifier, however, will develop power and, of course, the adjustmentsare such that the increment in aerial power due to the increment inpower from the quiescent amplifier is proportional to the increment inpower from the low power amplifier 2. During the negative cycle ofmodulation or when amplitude is less than carrier level the low poweramplifier 2 will provide decreased driving voltage to each of the twohigh frequency amplifiers 3 and 4, but this decrease will have no effecton the output from the quiescent amplifier since this does not functionduring modulation below carrier level. The output of the limiteramplifier 4, however, will be reduced and again the adjustments are suchthat this reduction is proportional to the decrement in power of the lowpower amplifier. In this way satisfactory modulation of the aerialcurrent is obtained while at the same time a high degree of efficiencyof the system when the carrier is unmodulated is also maintained. 1

Figure 2 shows another modification wherein the quiescent and limiterpower amplifiers are driven by unmodulated high frequency carrier,modulation being effected at each of these amplifiers.

Referring to Figure 2, i, is, as before, a source of high frequencyoscillations, i. e., a master oscillator, the output from which isapplied, after further amplification if desired, to two high poweramplifiers represented respectively at 3 and 4. The amplifier 3 is thequiescent power amplifier and the amplifier 4' is the limiter poweramplifier. Associated with each amplifier is a modulator valve 9 or If]the valve 9 being associated with the amplifier 3 and the valve H] withthe amplifier 4. One modulator valve 9 is so adjusted and arranged as tobe responsive only to the positive half cycles of modulating lowfrequency alternating potential while the other modulator valve I0 isresponsive only to the negative half cycles. The low frequencymodulation 'potentials are, applied to the grids of the valves 9 and H1via a common transformer 8 whose primary is connected to a source ofmodulating potentials or to a low frequency amplifier driven therefrom.The valves 9 and I0 are connected to effect modulation of the highfrequency oscillations at 9v and 4 in accordance with any well knownmethod, for example by the so-called series modulation method whereinthe anode to cathode impedance of the amplifier tubes in 3 and 4 are ina direct current circuit which connects a source of potential and theimpedances of the modulator tubes 9 and I0 in series. The modulatorassociated with the quiescent amplifier-is so adjusted as regardsnegative grid bias warm the absence of applied modulating voltagestheoutput of the said amplifier is zero or approximately zero while thegrid bias applied to the modulator associated with the limiter amplifierin so adjusted that in the absence of applied modulating voltages theanode-to-cathode drop in its associated modulator, i. e. in the valvelEI, is very small so that the whole or approximately the whole of theavailable anode potential is applied to the'amplifier 4 whose outputwill therefore be of maximum or approximately maximum value. Theamplifiers 4 and 3' receive anode potential as indicated from a commonsource H and of course the valves 9 and I9 are in series with thesevalves as is required for series modulation. It will be seen that sincein the carrier condition the quiescent amplifier will be taking littleor on power and the limiter amplifier will be absorbing only very smallpower, the adjustments may be such that a very high efficiency can beobtained in the carrier condition. The required action of the twomodulators is obtainable in any of a variety of different ways and inthe case illustrated in Figure 2 is obtained by including in the gridcircuit of the valves 9 and) rectifying devices H and 12 respectively,these rectifying devices being constituted for example by low resistancerectifiers such as mercury vapour diodes. As will be noted the diodesll, l2 are connected in opposite senses, that is, the grid circuit ofthe valve 9 is so connected that only voltages due to the positive halfcycles of the applied modulating wave can affect the grid voltage ofthis valve 9 while the diode i2 is so arranged that the valve I0 isaffected only by the negative half cycles. Suitable smoothing anddamping resistances and condensers may if desired be associated with thediodes H and I2. In certain circumstances, as will be apparent, therectifier in the grid circuit of the valve 9 may be dispensed withthough it is preferred to provide this rectifier.

The operation is as follows: During the positive half cycles ofmodulating low frequency potenial the rectifier II will pass current andreduce the negative bias upon the grid of the valve 9 thus permittingthe amplifier 3 to deliver energy to the aerial, but the rectifier 52 inthe grid circuit of the valve iflwill prevent any alteration in the biasof this valve and consequently the output from the amplifier 4 willremain unaffected. During the negative half waves of applied modulatingpotential the opposite effects take place. V The invention is notlimited to the particular arrangement herein disclosed and illustratedand obviously the invention is of general application to mostknown'circuit arrangements; for example the invention is not limited toarrangements wherein seriesmodulation is effected and'obviously thevarious valve amplifiers may be con stituted by triodes or valves havingmore than three electrodes while the various amplifiers may beconstituted byfsingle valves or a plurality of valves in parallel or inphase opposition or in any other convenient arrangementknown per se.

Further, it will be apparent that it is not funda-w mentallynecessarythat the two high frequency. high power amplifiers feed a common loadand; if desired, each amplifier may be arranged to en-: ergize aseparate aerial.

Having n'ow'particularly described andascer tained the-nature ofmy'said' invention and in what manner'the same is to'be'p'erfor'med', Ideclare that what I claimis:

1. A high frequency modulated "carrier wave transmitting systemcomprising a: source of "car rier waves, means'for modulating saidwaves, and

means for amplifying the modulated waves, said last means comprising twoelectron discharge device amplifiers, means for biasing the control gridof one of said devices whereby said one device amplifies only thoseoscillations whose am plitude is less than the carrier amplitude leveland means for biasing the control grid of the other device whereby thelatter amplifies only those oscillations which have an amplitude greaterthan the carrier amplitude level, and means for transmitting saidamplified modulated waves.

2. In a high frequency signal modulated carrier wave transmittingsystem, a source of high frequency oscillations, an output circuitcoupled with said source, two high power high frequency amplifierseffectively coupled in parallel relationship by separate input circuitsto said output circuit, limiting means associated with one of saidamplifiers for causing said one of said amphfiers to give a varyingoutput corresponding to modulated carrier wave variations abovesubstantially carrier Wave level only, limiting means associated withthe other of said amplifiers for causing the said other of saidamplifiers to give a varying output corresponding to modulated carrierwave variations below substantially carrier wave level only, the totaloutput corresponding to variations in modulated carrier wave over thewhole range of modulation, and means coupled between the output circuitsof said amplifiers for preventing reaction of either of said amplifierson the other.

3. A system in accordance with claim 2 including means for applyingmodulated carrier waves to said amplifiers.

4. A system in accordance with claim 2 including means for applyingunmodulated carrier waves to said amplifiers, and in which said limitingmeans associated with said amplifiers includes modulating circuitsindividual to each of said two amplifiers.

5. A high frequency modulated carrier wave transmitting systemcomprising a source of oscillations, means for modulating theoscillations therefrom, two high frequency high power amplifiers havingoutputs effectively coupled in parallel to an output circuit, means forapplying the modulated oscillations in parallel to the input circuits ofsaid two amplifiers, means for applying to one of said amplifiers anegative grid bias such that said amplifier is approximately quiescentat the carrier level, and means in circuit with the other amplifier forpreventing the same from passing current above substantially carrierlevel.

6. A high frequency modulated carrier wave transmitting systemcomprising a source of oscillations, two high frequency high poweramplifiers whose input circuits are effectively in parallel with respectto said source, a modulating circuit associated with each of saidamplifiers for modulating the oscillations therein, means for applyingmodulating potentials to the two modulating circuits, means for causingone of said modulating circuits to be effectively responsivesubstantially only to positive half waves of modulating potentials andmeans for causing the other modulating circuit to be effectivelyresponsive only to negative half waves of modulating potentialssubstantially as described.

7. A system in accordance with claim 6 characterized in this, that saidtwo modulating circuits comprise modulating electron discharge tubes.

8. A system in accordance with claim 6 characterized in this, that saidtwo modulating circuits comprising electron discharge tubes, one of saidmodulating circuits being effectively responsive only to positive halfwaves of modulating potentials while the other is effectively responsivesubstantially only to negative half waves of modulating potentials.

9. In a high frequency modulated carrier wave signalling system, themethod of operation which comprises generating oscillations, modulatingsaid oscillations in accordance with the message waves to betransmitted, transmitting the modulated waves over two paths, amplifyingonly modulated carrier waves of an amplitude below the mean carrierlevel in one of said paths amplifying only modulated carrier waves of anamplitude above the mean carrier level in the other of said paths, andcombining the resultant amplified waves for signalling purposes.

10. In a high frequency modulated carrier wave transmitting system, themethod of operation which comprises separately amplifying modulatedcarrier wave variations the amplitude of which is above the mean carrierlevel, separately amplifying modulated carrier wave variations, theamplitude of which is below the mean carrier level, and combining theresultant amplified waves in like phase relation.

WILLIAM THEODORE DITCHAM.

